The implementation and use of high voltage output systems within implantable cardioverter defibrillator (ICD) devices is well known. In order to generate the high voltage output necessary for effective defibrillation countershocks, a low voltage, high current battery system is connected via a transformer to a high voltage capacitor system. When a cardiac arrhythmia is detected by the ICD, the battery system quickly charges the capacitor system so that a high voltage defibrillation countershock can be delivered by the device. An example of the high voltage output system for an existing ICD device is described in U.S. Pat. Nos. 5,404,363 and 5,372,605.
Generally, ICD devices have high voltage capacitor systems comprised of aluminum electrolytic capacitors and high current battery systems comprised of silver vanadium oxide (SVO) battery cells. Although aluminum electrolytic capacitors have a relatively high energy density per volume, these type of capacitors tend to degrade electrochemically over time thereby increasing the charge time required to fully charge the capacitor system. The SVO battery cells also have a tendency to degrade electrochemically over time due to the increased equivalent series resistance (ESR) within the battery which decreases the current output capabilities of the battery.
The conventional solution to both of these problems has been to conduct a periodic reforming of the high voltage output system of an ICD by charging the capacitor system to its full rated voltage and then allowing that voltage to slowly trickle off. In this way, both the high current battery system and the high voltage capacitor system are exercised so as to reform the electrochemistries of each system, thereby reducing the impact on charge performance and component life due to electrochemical degradation over time. Originally, this reforming process was accomplished manually by having a patient visit the physician every two to three months, at which time the physician would program the ICD to charge, but not deliver, a full voltage rated countershock. Presently, the reforming of the high voltage output system is accomplished automatically by the ICD based on a fixed time period (e.g., every month, every six months), at the end of which a full charge cycle of the capacitor system is automatically conducted. While this kind of simple periodic reform cycle was more than effective for early ICD devices where the life span of the device was typically less than three years and the battery budget could easily support the periodic reform cycles, newer ICD devices are smaller and have much longer life spans. An example of such an ICD which is used as a prophylactic device is described in U.S. Pat. No. 5,439,482. In these newer designs for an ICD, battery power is at more of a premium than in previous designs and the periodic reforming of the high voltage output system can represent a significant portion of the battery budget over the life of the device.
Two alternate techniques for accomplishing reforming of the battery system and the capacitor system are disclosed in the previously-identified co-pending applications. In the first application entitled "AUTOMATIC CAPACITOR MAINTENANCE FOR AN ICD", a technique is disclosed for measuring the leakage current of the capacitor system at a relatively low voltage and using this value to estimate whether the capacitor system needs to be reformed. By utilizing a low voltage test process, battery power is conserved and full voltage reforming is conducting only when it is determined that the capacitor is in need of reforming. In the second application entitled "AUTOMATIC BATTERY MAINTENANCE FOR AN ICD", a technique is disclosed for measuring an electrical parameter of the battery system and using this value to determine whether the battery system needs to be reformed. Again, battery power is conserved by only performing a full voltage reform when it is determined that the internal resistance of the battery system has increased to the point where charge performance is degraded. While each of these inventions represent a significant improvement over the existing periodic reform technique, both of these inventions suffer from the disadvantage of potentially requiring additional circuitry within the ICD in order to implement the invention.
Although existing techniques for reforming the high voltage output system of an ICD are adequate for current ICD systems, it would be advantageous to develop a more efficient reform system for the high voltage output system of an ICD. It would also be advantageous to develop a reform system that did not require significant additional circuitry within the ICD.